Passive solar design makes use of the sun’s natural energy for the heating and cooling of living spaces. In this approach, the building itself, or some element of it, takes advantage of natural energy characteristics in materials and air created by exposure to the sun. As opposed to active solar heating systems, which use electricity to transfer and distribute solar-heated air or liquid into the home, passive systems are simple, have few moving parts, require minimal maintenance, and require no mechanical systems.

Passive solar design has often been discounted as a viable energy alternative in northern latitudes such as Maine, where the angle of the sun is lower and winter daylight hours are limited. This is sometimes because of the additional cost of reducing heat loss by making the home more energy efficient. However, in a well-designed or -retrofitted home, the energy of the sun can provide a significant portion of a Maine home’s heating needs.

As fuel costs rise, many new homes are being built with increased insulation and other features to improve energy efficiency. Also, many existing homes are being retrofitted with energy-saving improvements. Once energy-efficient features are in place, the use of passive solar energy becomes more viable.

The most efficient and least expensive way to tap the sun’s energy is through the design and construction of houses that collect and store solar energy without fans, pumps, or other mechanical devices. Passive-solar building design captures the sun’s heat and takes advantage of the natural processes of reflection, radiation, conduction, and convection to provide uniform temperatures throughout the structure.

The solar energy resource

Passive solar heating, especially when used in conjunction with energy-efficient construction, can significantly reduce dependence on costly fuels. For example, two identical houses, sitting side by side and facing south, will receive the same amount of solar energy. However, if one is more heavily insulated than the other and has fewer cracks where air can leak in and out, it will retain more heat, and require less fuel over the heating season. In fact, insulation may well be the most important element of passive solar design.

Insolation (incoming solar radiation) is usually measured in terms of the number of BTUs (British thermal unit—a measure of heat energy) striking a square foot of surface during a specified time period. The amount of insolation received at a given location in a day is dependent upon the area and thickness of cloud cover as well as the sun angle and the number of hours of available sunlight. Because of the interplay of these factors, insolation statistics do not necessarily correspond directly with latitude.

For efficient passive solar heating, a house must serve as a 1) solar collector, 2) heat storehouse, and 3) heat trap.

The house as a solar collector

Orientation and solar gain

Every surface of a building that is directly exposed to the sun’s rays is collecting solar energy. Surfaces that are not directly exposed to the sun’s rays can be heated by convection, conduction, and radiation. You can maximize this collection in your passive solar house when you build or retrofit with the following considerations in mind.

Siting considerations—During the heating season, the sun’s path makes an arc in the southern sky. When designing and constructing a passive solar house, you should take into account the locations of trees, other houses, and mountains that might stand between the house and the sun’s path in the sky. These objects can create shadows on a building and reduce solar collection for that section of the house.

House orientation and shape—In the northern hemisphere, south sides of houses receive the most solar radiation during the winter. East and west sides receive more solar radiation in summer than in winter. When designing a passive solar house, make the south side of the house longer than the east/west side. But don’t build a long, one-story ranch style house because this shape requires the most surface area of roof and walls for a given amount of living space. Since heat loss is a function of exposed wall and roof area, reducing those areas to a minimum reduces the heat load for the structure.

Window placement—South-facing glass windows allow direct sunlight to heat the house interior. In an energy-efficient house, south-facing windows can provide up to 30 percent or more of the heating load. You can add an overhanging eave or awning on south-facing windows to prevent overheating during the summertime. Avoid having too much glass on the west side of the house, where the low evening sun hangs for hours and can easily overheat rooms that have already been warmed all day by the southern sun.

Glass design—Maximize the R-value (a measure of resistance to heat flow) of windows without inhibiting visibility/light. There are a number of new types of windows on the market, including those with low emissivity or “low-e” glass. Low-emissivity glass decreases radiant heat loss to the outside and increases R-value, generally without markedly lowering visibility/incoming light.

Solar greenhouse—When attached to a south wall, a solar greenhouse provides additional heat collection area as well as space for houseplants and food production in winter months.

The house as a heat storehouse

An energy-efficient house must effectively store its passively gained solar heat. One practice in passive solar design is to include an indexed amount1 of what is termed thermal mass2 in the home. This typically is accomplished by designing the solar-gain space (the space adjacent to south-facing windows) with a large amount of concrete masonry or other dense material that can hold heat. The role of thermal mass is to store the solar heat during the day, preventing overheating, and release it back to the living space at night when the sun has set.

A rule of thumb for passive solar design that recommends one cubic foot of concrete for every square foot of solar aperture area (sunlight-admitting glass) was developed in the southern and southwestern U.S. The inclusion of thermal mass, such as a concrete, brick, stone, or containers of water, should be thoroughly investigated for any structure being designed or retrofitted for passive solar heating.

During retrofitting, the installation of adequate thermal mass may not always be feasible for a variety of reasons, including insufficient space for an adequate volume of material, and insufficient load-bearing capability to support heavy thermal-mass material. This should not be considered a reason to avoid passive systems. Fuel savings from passive systems can be significant even if thermal mass is limited.

Effective passive solar heating can be obtained by combining south-facing glazing (glass) with a thermally efficient building envelope or shell.

The house as a heat trap

An important part of efficient heating is preventing heat from escaping the house. Successful passive solar heating is dependent on good building, insulation, and conservation practices. High heat loss through walls, ceilings, and windows increases both the area required for solar energy collection and the amount of additional energy sources needed.

The recommended minimum insulation values for a passive solar house are R-30 walls and R-50 ceilings. However, any increased insulation will increase the effectiveness of solar heating because of reduced heat loss.

Well-insulated walls and roofs, along with appropriate sizing and placement of windows, can cut fuel bills by 50 percent or more. Single-paned windows lose three times as much heat as triple-paned windows, and 40 times as much as an R-40 wall. Many new types of windows, including low-emissivity windows, increase the insulating capacity of glass and can make the house more comfortable. Use of insulated shutters can reduce nighttime heat loss.

Most of us are familiar with R-value, which measures insulating value—resistance to heat transfer. Windows, however, are rated by U-value, which is the inverse of R-value. U-value measures the rate of heat transfer through a substance—its conductivity. If you were buying insulation, you would want a product with a high R-value. When buying windows, you want a product with a low U-value, indicating that you will lose less heat through your windows. In Maine, you would want windows with a U-value of 0.35 or less.3 Low-e glass is appropriate for all but the south-facing windows of the house. The energy-collecting windows on the south side of the house should have windows with a high solar heat gain coefficient (SHGC) rating.

Economics

Depending on the elements used, the builder’s familiarity with the concept, and, more importantly, the small details of passive solar design and construction, passive solar features can increase design and construction costs by 0 to15 percent. However, this is a one-time cost for energy-saving features that last the lifetime of the building.

Using passive solar home design software and investigating newly introduced building products can help minimize the extra cost of solar design. Features such as proper siting, house orientation and shape, and window placement don’t increase costs; they simply require you to become more aware of the surrounding environment.

There are also a variety of federal and state tax incentives available. Consult the Database of State Incentives for Renewables and Efficiency at www.dsireusa.org.

Passive solar design provides space heat that is inherently simple, clean, safe, and cheap. It is an investment that continues to pay throughout the life of your home.

Seifert, R.D., and Mueller, G.S., 1983. An Analytical Study of Passive Solar Energy and Mass Storage: Observations from A Test Building in Fairbanks, Alaska. Research Report to Fairbanks: Alaska Department of Transportation Public Facilities, Research Section. 42 pp. plus appendices.

1 Often described as the “glass to mass ratio.”2 In passive solar design, thermal mass is used to describe dense material with the ability to store the heat produced by sunlight.3 U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy, “Windows,” Energy Savers (2006). http://www1.eere.energy.gov/consumer/tips/windows.html

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